Ensure all tools (where possible) produce native 'merged' tables

[jfy133]

Description of feature

Where possible, in addition to our standardisation tool with taxpasta, we should also try and support 'native' OTU tables, as we already have for mOTUs:

Tool	Merge Tool	Module	Done
Bracken	combine_bracken_outputs.py	https://github.com/nf-core/modules/pull/2022	[needs bracken first]
Centrifuge	combine_kreports.py [Can use same as krakentools?]		☑️ https://github.com/nf-core/taxprofiler/pull/130
DIAMOND	❌
Kaiju	kaiju2table (we currently don't merge multiple samples - done!)	https://github.com/nf-core/taxprofiler/pull/126	☑️ https://github.com/nf-core/taxprofiler/pull/126
Kraken2	combine_kreports.py via krakentools	https://github.com/nf-core/modules/pull/2018	☑️ https://github.com/nf-core/taxprofiler/pull/130
KrakenUniq	❌ [breaks too easily]
MALT	❌ None by default, I have a 'custom' script [here](https://github.com/jfy133/rma-tabuliser EDIT: this only works in very specific cases with a special awk script so I decided best not )
Metaphlan3	https://github.com/biobakery/biobakery/wiki/metaphlan3#merge-outputs	https://github.com/nf-core/modules/pull/2026	☑️ https://github.com/nf-core/taxprofiler/pull/136
mOTUs	motus-merge (TODO: update to have meta once https://github.com/nf-core/modules/pull/2068 is in!)	https://github.com/nf-core/modules/pull/1734	☑️ https://github.com/nf-core/taxprofiler/pull/137

I also have a rough script for MALT/MEGAN rma2info output....

[jfy133]

Will require a re-structuring for clarity (I think).

i.e.

results/tool/db/sample_1/*txt
results/tool/db/sample_2/*txt
results/tool/db/sample_3/*txt
results/tool/db_combined.txt

Working out how to do this for Kaiju

[jfy133]

Note mOTUs merge will need to be checked